Bacillus anthracis, the causative agent of inhalation, cutaneous, and gastrointestinal anthrax, is an aggressive pathogen, whose unique properties make it an ideal bioterrorism agent. After inhalation, B. anthracis spores germinate in alveolar macrophages, which carry the bacteria to lymph nodes where they replicate and eventually disseminate through the blood stream. During this process, B. anthracis kills the macrophage and thereby evades detection and attack by the host innate immune system. Macrophage killing is mediated by the lethal toxin (LeTx), whose active subunit is lethal factor (LF). Curiously, at very low concentrations, LF activates the macrophage to produce proinflammatory mediators, whereas at higher concentrations it displays selective cytotoxicity toward macrophages. We found that modest concentrations of LF selectively induce the apoptosis of activated but not resting macrophages. This process is likely to depend on the ability of LF to cleave MAPK kinases (MKKs ) at a site required for MAPK activation. Our results suggest that activation of p38 MAPKs together with the activation of NF-(B is necessary for prevention of activation-induced death of macrophages. We plan to test the hypothesis that LF specifically kills activated macrophages by targeting this anti-apoptotic mechanism. We also plan to better understand why at low concentrations LF activates, rather than kills, macrophages. We therefore plan to pursue the following specific aims: 1) Determine the signal transduction mechanism by which very low concentrations of LF induce macrophage activation and production of proinflammatory cytokines; 2) Determine the mechanisms by which modest concentrations of LF induce the apoptosis of activated macrophages; 3) Identify which NF-(B target genes protect activated macrophages against LF-induced apoptosis and which genes are induced in response to very low, non-cytotoxic, concentrations of LF to mediate inflammatory shock; and 4) Identify the mechanism for activation-induced apoptosis of macrophages that lack NF-(B or p38 activity. In addition to providing a much better molecular understanding of the interaction between B. anthracis and its host, this project should enable the design of new therapeutic strategies that will tilt the balance in favor of the host macrophage in the battle against B. anthracis and inhalation anthrax.